Stress migration becomes a crucial issue for copper (Cu)/low dielectric constant (low-k) interconnects where a low dielectric constant is less than about 3.0. The usual test structures, such as pull-back (small lower metal with a large top metal), hump (large lower metal with a small top metal) patterns and landed electromigration (EM) patterns, etc., can only give a combination of the line and via resistance shift post stress.
The current best practice is to set up a set of line patterns with the dimensions as close as those in the typical stress migration (SM) patterns. However the result still varies depending upon the position on the wafer and the local pattern density near the test patterns.
U.S. Pat. No. 6,342,733 B1 to Hu et al. describes a test structure for EM and SM.
U.S. Pat. No. 6,004,827 to Ryan describes various test structures with aluminum runners and overlying dielectrics.
U.S. Pat. No. 5,930,587 to Ryan describes an SM method and test structure.
Accordingly, it is an object of one or more embodiments of the present invention to provide an improved stress migration test structure and stress migration method used to measure and differentiate between line stress migration and via stress migration.
Other objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a metal line having a middle and opposing first and second ends is formed. First and second opposing pads electrically connected to the respective opposing first and second ends of the metal line through respective first and second step-width line structures are formed. A third pad connected to the metal line proximate its first end by a first via through a first metal structure is formed. A fourth pad connected to the metal line proximate its second end by a second via through a second metal structure is formed. The first and second vias are equidistant from the respective first and second ends of the metal line. The stress migration of the first via is determined by measuring the: sheet resistance between the first pad and the third pad; and/or the stress migration of the second via is determined by measuring the sheet resistance between the fourth pad and the second pad. And the test pattern structure thereof.